Recent legislation in Europe and North America has been imposing more stringent exhaust emissions requirements for vehicles. In particular, there have been new mandates for lean-burn technologies for compression ignition (diesel) engines, as well as direct injection engines that must operate under lean and ultra-lean conditions. Lean burn engines exhibit high levels of nitrogen oxide (NOx) emissions that are difficult to treat in oxygen-rich exhaust environments characteristic of lean-burn combustion. Exhaust after treatment technologies are currently being developed that will treat NOx under these conditions. One of the technologies comprises a catalyst that facilitates the reactions of ammonia (NH3) with the exhaust nitrogen oxides (NOx) to produce nitrogen (N) and water (H2O). This technology is referred to as selective catalytic reduction (SCR).
However, one of the obstacles is that ammonia (NH3) is difficult to handle in its pure form in the automotive environment; therefore, systems have been developed that use a liquid aqueous urea solution, typically at 32% concentration of urea (CO(NH2)2). The solution is referred to as AUS-32, also known under its commercial name of AdBlue®. The urea solution is delivered to the hot exhaust stream of a vehicle and is transformed into ammonia in the exhaust after undergoing thermolysis or thermodecomposition into ammonia and isocyanic acid (HNCO). The isocyanic acid then undergoes a hydrolysis with water present in the exhaust and is transformed into ammonia and carbon dioxide (CO2). The ammonia resulting from the thermolysis and hydrolysis then undergoes a catalyzed reaction with nitrogen oxides as described previously.
One of the obstacles with using a urea solution is that the current guidelines for AUS-32 contain a mixture of urea with water, which means that the solution is subject to freezing, typically at minus 11° C. This prevents an obstacle with SCR technologies because there is a substantial concern about the freezing of the solution which can cause expansion and blocking of the reductant delivery unit (RDU) that delivers the urea solution to the exhaust system. If the solution freezes within the RDU, the whole RDU can become clogged or have components such as hoses, injection heads, tanks, pumps and other components break due to the expansion of the frozen solution.
In order to address the freezing concerns, several different types of purge systems have been developed where the solution supply line and injection head of the RDU are purged upon turning off the vehicle engine, so that the components will have very little if any solution within, thereby helping to eliminate the potential for damage or blockage due to freezing of the solution. Additionally, there have been other developments where heaters are placed within the solution supply tank and heat tape is used along the solution supply line to ensure that the solution supply line remains warm and does not allow the solution to freeze. However, these solutions rely upon providing power to the RDU system components in order to heat the components when the vehicle is sitting with the engine off. If the vehicle is stored for a long period of time in cold weather such systems can cause an unwanted drain on the vehicle's battery or require a separate power source. It is therefore desirable to develop more practical freeze protection for SCR systems in order to protect the components of the RDU.